That is good news. Are there any ponds in your area and if so what do they do? We have much the same water run off system that you do, over time nature has decided the best route and established paths for the water to follow. Here the waterways are government property so there is little fear of someone blocking them, new builders channel their runoff into a nearby waterway.
Is it possible to use these established waterways? Maybe use more than one to reduce the amount of runoff?
My pond is smaller than yours and I get over 6 and 1/2 feet of rain a year, most of which falls in 3 months. My pond overflows during a 3 month period, but even in heavy rain the overflow is easily managed with an 18" wide spillway, I have never seen it over an inch deep. I have very little run off so the water is almost all direct rainfall. I overflow into a waterway that runs all year round and my contribution is barely noticed. Additionally, I lose about an inch a day to evaporation and seepage. I gain about a half inch a day from ground water.
I know that doesn't help much I am just trying to give you an idea of the size of your problem. Not counting the government. I checked google earth and what I saw was well photographed.
